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Bt Toxin in Insect-Resistant Crops
Overview Bt toxin is a delta endotoxin produced by the bacterium Bacillus thuringiensis, a gram-positive soil dwelling bacteria. Delta endotoxins have natural insecticidal properties due to their ability to cause cell lysis in the epithelial cells of insects with alkaline intestinal tracts - thus killing the insect. Cell lysis occurs when the ingested proteins are activated through proteolytic cleavage; when these endotoxins bind to the epithelial cells within the insect's intestinal tract, cation-selective channels are formed which cause the cell to lyse - effectively paralyzing the digestive system. The affected insects will die within a few days due to starvation and or blood poisoning. These delta toxins are produced by Bacillus thuringiensis during the process of sporulation in the form of crystals. They are also found naturally in the stomachs of certain types of caterpillars, moths, and butterflies. However, the toxic properties are only activated when exposed to an alkaline (basic) pH; therefore, the protein will only affect target pests with alkaline intestinal tracts but will remain completely non-toxic to other organisms. As such, Bt is the only microbial insecticide that is used on a large scale. = Usage Insecticide Powder Bt toxin may be purchased in a dried powder and used as a spreadable insectiside. Advantages * The toxic crystals only become activated within an alkaline digestive tract (pH of +7) and will only harm insects with this specific trait. * Does not kill beneficial insects (ie. honeybees, butterflies) due to their non-alkaline digestive tract. * Does not harm humans, pets, and other natural predators of insects. * May be used on crops. Disadvantages * Susceptible to degradation in sunlight - may only last a week or less and fail to remove the presence of pests during the effective time of application. * Does not kill rapidly - takes time for the insect's intestinal tract to lyse and cause blood poisoning/starvation. * May disrupt learning performance of certain insects (ie. bees). * Shorter shelf life - effectiveness of this powered insecticide decreases after two to three years. Genetically Modified Plants Transgenic plants that have benefited from this added toxin include potato, corn, cotton, and tobacco plants. "The Bt genes engineered into crops and approved for release include, singly and stacked: Cry1A.105, CryIAb, CryIF, Cry2Ab, Cry3Bb1, Cry34Ab1, Cry35Ab1, mCry3A, and VIP, and the engineered crops include corn and cotton."http://en.wikipedia.org/wiki/Bacillus_thuringiensis#Insect_resistance Targeted Species (List provided by Colorado State University) Kurstaki'' Strain (Biobit, Dipel, MVP, Steward, Thuricide, etc.)''' * Vegetable Insects: * Cabbage Worm (cabbage looper,imported cabbage worm, diamond back moth, etc.). * Tomato and tobacco hornworm. * Field and Forage Crop Insects: * European corn borer (granular formulations have given good control of first generation corn borers). * Alfalfa caterpillar, alfalfa webworm. Fruit crop insects * Leafroller * Achemon sphinx Tree and shrub insects * Tent caterpillar. * Fall webworm. * Leafroller. * Red-humped caterpillar. * Spiny elm caterpillar. * Western spruce budworm. * Pine budworm. * Pine butterfly. '''Israelensis' strains (Vectobac, Mosquito Dunks, Gnatrol, Bactimos, etc.)' * Mosquito. * Black fly. * Fungus gnat. '''San diego'/tenebrionis strains (Trident, M-One, M-Trak, Foil, Novodor, etc.) * Colorado potato beetle. * Elm leaf beetle. * Cottonwood leaf beetle. Recombinant DNA Construct & Purification Process In order to incorporate the insecticidal properties of Bt toxin into the crop of choice, the Bt gene that produces the desired crystal product must first be isolated and joined to a marker gene to create antibiotic resistance. This is accomplished through splicing the gene and the marker into a vector. This vector is then inserted into a host cell where it may reproduce and make additional copies of itself. These cells are grown in the presence of the specific antibiotic tied to the Bt gene to ensure that only the cells carrying both the desired Bt gene and the gene conveying antibiotic resistance are able to survive in this environment. The surviving cells grow into the desired crop, which now contains the Bt gene and is resistant to certain insects. References Recombinant DNA Bacillus thuringiensis & Colorado State University Delta endotoxins Bt in Biotechnology